1. Field of the Invention
The present invention relates to the field of digital systems, in particular, digital systems having multiple state machines. More specifically, the present invention relates to a method and apparatus for resolving multiple asynchronous control signals to a stable, predictable, and useful output signal.
2. Background
In a number of digital applications, it is often necessary to resolve multiple asynchronous control signals. For example, in a data instrumentation system comprising a number of trace boards for monitoring, acquiring data and controlling a number of emulation processors operating at different speeds, it is desirable to have a signal that can be controlled by each state machine on the trace boards, and can also be sampled by the state machines. To do so, it is necessary to resolve the asynchronous control signals originating from each state machine to create this signal. Traditional prior art methods for resolving asynchronous signals typically involve synchronizing the input signals to a common clock first. Synchronizing the control signals to a predetermined common clock has the disadvantage of having a delay cost of at least two clocks for the synchronization logic. In a digital system such as a data instrumentation system where a number of data values are being acquired, this delay can represent a large lag in information from the original event that created the control signals. Additionally, to guarantee detection of all control signals, the predetermined common clock should be at least twice as fast as the fastest control signal. This places a very difficult performance requirement on the circuit and effectively limits the top operating speed. Thus, it is desirable to be able to resolve asynchronous control signals to a stable, predictable, and useful output signal without having to first synchronize all the control signals to a common clock. As will be disclosed, the present invention provides such a method and apparatus, which advantageously achieves the desirable results. As will be obvious from the descriptions to follow, the present invention has particular application to high speed data instrumentation systems.